Bring the apple cider to a boil over medium heat, then lower the heat and reduce it to 3/4 cup (180 ml). Reserve 1/2 cup (120 ml) for the batter and 1/4 cup (60 ml) for finishing. Let cool.
Melt the 8 tablespoons of butter over low heat. Mix the flour, cornstarch, baking powder, baking soda, salt, cinnamon and nutmeg in a separate bowl.
Mix the cooled 1/2 cup of reduced cider with the sour cream and vanilla. In another bowl, whisk the eggs and granulated sugar for about 2 minutes until pale yellow and frothy, then whisk in the melted butter.
Add the dry ingredients in three additions, alternating with the cider–sour cream mixture in two additions. Mix until combined.
Butter and parchment-line a 9 × 5-inch (23 × 13 cm) loaf pan. Fill with batter and bake in a preheated oven at 325°F (160°C) for 60–80 minutes, until a toothpick comes out clean. The cake in the video took 60 minutes.
Poke the baked cake with a toothpick and spoon over 3 tablespoons of the reserved reduced cider. Leave the final tablespoon for the coating.
Mix 1 tablespoon melted butter with the remaining tablespoon of reduced cider. Unmould the cake and brush the top and sides with this mixture.
Combine the sugar, cinnamon, nutmeg and pinch of salt listed for the topping. Sprinkle over the top and coat the sides of the cake.